Tunable resonant frequency kinetic energy harvester

Abstract

An energy harvester comprising: a substrate; two magnets coupled to the substrate in close proximity to each other with like magnetic poles facing each other creating a flux gap; a coil coupled to the substrate and disposed within the flux gap, wherein the coil and the magnets are coupled to the substrate such that substrate acceleration causes relative elastic motion between the magnets and the coil thereby exposing the coil to a changing magnetic flux; and a resonant frequency tuner coupled to the substrate and configured to adjust the resonant frequency between the coil and the magnets.

1. An energy harvester comprising:
 a substrate; 
 two magnets coupled to the substrate in close proximity to each other with like magnetic poles facing each other creating a flux gap; 
 a coil coupled to the substrate and disposed within the flux gap, wherein the coil and the magnets are coupled to the substrate such that substrate acceleration causes relative elastic motion between the magnets and the coil thereby exposing the coil to a changing magnetic flux; 
 a proof mass elastically coupled to the substrate via elastic members, wherein the coil is coupled to the proof mass and the proof mass is elastically coupled to the substrate and wherein the magnets are rigidly coupled to the substrate, wherein the proof mass comprises top and bottom surfaces with top and bottom recesses respectively therein, and the coil comprises top and bottom coils, both coils being wound in the same direction, and wherein the top and bottom coils are mounted in the top and bottom recesses respectively so that both the top and bottom coils are positioned approximately in a null of a magnetic field gradient created by the magnets; 
 a resonant frequency tuner coupled to the substrate and configured to adjust the resonant frequency between the coil and the magnets; and 
 a force feedback system operatively coupled to the resonant frequency tuner and an output of the top and bottom coils, wherein the force feedback system is configured to monitor the output of the top and bottom coils and adjust the tension in the elastic members via the resonant frequency tuner in real time such that at any given vibration frequency of the substrate, the proof mass vibrates at a resonant frequency with respect to the magnets. 
 
     
     
       2. The energy harvester of  claim 1 , wherein the resonant frequency tuner comprises a hermetically-sealed enclosure filled with a gas and further comprises a gas pressure adjuster, wherein the coil and the magnets are enclosed in the enclosure. 
     
     
       3. The energy harvester of  claim 1 , wherein the resonant frequency tuner is a tunable load-matching circuit, wherein the load-matching circuit is configured to change an electrical load external to the coil by tuning the impedance of the load-matching circuit. 
     
     
       4. The energy harvester of  claim 1 , wherein the resonant frequency tuner is a circuit comprising a switch, wherein the circuit is configured to switch connection to and from the coil with a given duty cycle, thereby altering the resonant frequency of the energy harvester. 
     
     
       5. The energy harvester of  claim 1 , wherein the resonant frequency tuner is configured to alter the mass of the proof mass. 
     
     
       6. The energy harvester of  claim 1 , wherein the elastic members are made of a magnetic material and the resonant frequency tuner comprises a permanent magnet that is adjustably coupled to the substrate such that the position of the permanent magnet with respect to at least one of the elastic members is adjustable. 
     
     
       7. The energy harvester of  claim 1 , wherein the elastic members are springs and the resonant frequency tuner is a mechanical spring-tension adjuster operatively coupled to the springs. 
     
     
       8. The energy harvester of  claim 1 , wherein the resonant frequency tuner is configured to alter the spring constant of the elastic members by effectively altering the length of the elastic members. 
     
     
       9. The MEMS energy harvester of  claim 1 , wherein the elastic member is a compliant region of the substrate. 
     
     
       10. The MEMS energy harvester of  claim 9 , wherein the resonant frequency tuner comprises an electrode integrated into the substrate near the elastic member such that when a potential is applied between the elastic member and the electrode an electrostatic force is created which alters the tension of the elastic member. 
     
     
       11. The MEMS energy harvester of  claim 10 , wherein the elastic member forms a cantilevered beam with the proof mass supported at the end thereof. 
     
     
       12. The MEMS energy harvester of  claim 9 , wherein the substrate forms a hermetically-sealed enclosure containing the coil and the magnets, and wherein the enclosure is filled with a gas, and wherein the resonant frequency tuner comprises a gas pressure adjuster. 
     
     
       13. A micro-electro mechanical system (MEMS) energy harvesting apparatus comprising:
 a substrate; 
 two magnets, each magnet having an opposing face, wherein the magnets are rigidly coupled to the substrate such that the opposing faces are substantially parallel to each other with like magnetic poles facing each other with a space there-between so as to form a steep flux gradient region in and around the space between the opposing faces; 
 an elastic member having first and second ends, wherein the second end is coupled to the substrate; 
 a proof mass coupled to the first end of the elastic member such that the proof mass is positioned within the steep flux gradient region and is configured to move with respect to the magnets responsive to substrate acceleration; 
 a coil coupled to the proof mass so that the coil is exposed to a changing magnetic flux arising from motion of the proof mass with respect to the magnets; and 
 a resonant frequency tuner operatively coupled to the elastic member; and 
 a force feedback system operatively coupled to the resonant frequency tuner and an output of the coil, wherein the force feedback system is configured to monitor the output of the coil and adjust the tension in the elastic member via the resonant frequency tuner in real time such that at any given vibration frequency of the substrate, the proof mass vibrates at a resonant frequency with respect to the magnets.